It's very hard to gauge when it will be crowded vs empty. E.g. even if you go during a school week, it could be a holiday/etc for another country, or Disney could be hosting a festival or marathon.
Look at a crowd calendar online for estimations of crowd levels https://www.undercovertourist.com/orlando/crowd-calendar/
See https://www.mousesavers.com/2020-pop-century-room-rates-season-dates/ for an example of room rate seasons. Typically rates are cheapest during non-peak weeks.
Restaurant reservations open 60 days in advance, so ideally you should have your basic itinerary planned out at least two months in advance
You don't have to reserve that far in advance, but be aware that the specific restaurant or time you want might not be available
If you can't initially get the reservation you want, use the Touring Plans Reservation Finder to get an email/sms alert if the reservation becomes available https://touringplans.com/walt-disney-world/reservation-finder
Disney now requires Park Reservations for each day, so you must pre-plan and reserve your specific park for each day. Typically reservations are easy to get in advance, but parks (especially Magic Kingom) do "sell out" during peak times.
With the new Genie+ / Lightning Lane system, you can no longer book ride reservations in advance like you could with FastPass+. Ride reservations can be only be made on the day of.
Stay on-site at an official Disney hotel. They are more expensive than off-site hotels but the convenience and "magical" factor are worth it
Free frequent buses to the disney theme parks (if you drive from an offsite hotel you pay for parking at the parks)
Extra magic hours (at certain hotels) let you have a few extra hours in the park vs people staying offsite
Value vs Moderate vs Deluxe
Value resorts are fine, but not really good for lounging/relaxing. Moderates and Deluxe have more theming/bars/activities to lounge.
Value resorts require buses for all transportation (except Pop Century), vs most moderates and deluxes have an alternate form of transport (boat, monorail, skyliner or walking) to at least one park/springs.
The Skyliner is really good. Staying at a resort with access (e.g. Pop Century, Art of Animation, Caribbean Beach, Riviera) is definitely worth it over taking busses everywhere.
You can stay at a value hotel then take the free buses over to a deluxe to enjoy them without the big price tag.
There aren't direct hotel-to-hotel buses. You'll need to take a bus to a park or Disney Springs first then take another bus to the other hotel. (Or just Uber)
Don't plan a 4 day trip doing one park each day and try to see/ride everything. You will be exhausted.
Try to have at least 1 "free" day in the middle of the trip to leisurely visit Disney Springs or visit other Disney hotels.
A longer Disney trip can actually be less exhausting, because you have more time to relax.
Cheapest tickets are at https://boardwalkticketing.com/walt-disney-world-base-tickets/ (Note only 3+ days are discounted. For 1-2 days you must purchase directly through Disney)